4. comp sci - ijcse - damage detection using a non-destructive maysoon h. al-moswi iraq

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DAMAGE DETECTION USING A NON-DESTRUCTIVE TECHNIQUE FOR INSPECTION

WIND TURBINE BLADES

MAYSOON H. AL-MOSWI Informatics Center for Research & Rehabilitation, Kufa University, Najaf, Iraq

ABSTRACT

The increasing of the use renewable power sources in the next decades which make the use of non-destructive

testing techniques will improve the regular inspections of wind turbine blades. There are several techniques for non

destructive testing such as thermography, x-ray imaging, acoustic emission vibration analysis and ultrasonic testing. In this

paper we evaluated the measurements of the propagation characteristics of waves that are used to determine material properties; this technique is useful for detection of flaws to discover damages at the boundary surfaces. The ultrasonic

A- scan and C-scan imaging are used for the area to map interface disband. This method of testing object be carefully

planned with regard to safety economic and high efficiency and investigated during the pulse – echo technique developed

for practical applications.

KEYWORDS: Ultrasonic, Echo, Technique, Wind Turbine Blade, NDT Techniques, A-Scan, C-Scan

INTRODUCTION

Ultrasonic (NDE) non-destructive Evaluation is one of the most widely methods today to examine an object,

material or system as a way of finding out the nature of a solid material-its thickness, its flaw, its elasticity and more this

knowledge has many applications in aircraft, piping, semiconductor fabrication, railroad and other industries [1].

An ultrasonic NDT has been used to test wind turbine blades. This technique is based on ultrasonic waves which might

have higher frequency more than 20khz. are generated and propagate into a material by a transducer to transmit and receive

the reflecting signal at the same time to test the material’s composition, structure, elastic properties, density and

The blades are important components in wind turbines, so they are needed to be investigated and supervised

regularly. The sample which has been inspected for the sake consists of two layers; the top gel-coat layer

(varies in thickness between 1mm and 3mm), the bottom layer is glass-reinforced plastic (GRP) which has a constant

thickness of 4mm.

In this paper, ultrasonic NDT techniques, suitable for testing of wind turbine blade is used for inspection flaws

between the GFRP layers appear like delamination or lake which is the most technology applied for condition monitoring.

Ultrasound – Echo Technique

In principle, the pulse technique is the simplest of all the ultrasonic testing methods, and it is the one most

commonly used, it consists basically of measuring the time taken for short train of sound waves to move through a given

distance [3].

This distance can determine if the speed of sound in the material is known, or if the distance be known, thenseeped of sound can rapidly calculate by measuring the relative amplitude of pulses which have travelled different

distance, which can determine the attenuation coefficient. The principle is shown in Figure (1) is very simple.

An ultrasonic pulse is sent into the test specimen using an ultrasonic transducer.

International Journal of Computer Science

and Engineering (IJCSE)

ISSN(P): 2278-9960; ISSN(E): 2278-9979

Vol. 3, Issue 1, Jan 2014, 29-38

© IASET



30 Maysoon H. Al-Moswi

Figure 1: Principle of an Ultrasound-Echo Measurement

The waves are traveling through the specimen and are reflected at the back wall or at flaws within the material.

The reflected waves are recorded using the same transducer. A longer travel path results in a later echo [2].

The location of the back wall echo therefore gives information about the thickness of the inspected structure.

This technique will be used to detect areas of missing bond between the belt and the bars within the wind turbine blades.

The ultrasonic waves have to travel through several centimeters of GFRP, which is due to the fibers a highly

sound scattering and damping material. Therefore a high voltage ultrasonic pulse is used to send enough energy into the

material. The wind turbine is fast growing and very source of environmentally safe and renewable energy with a high

potential. It is necessary to perform continuous condition monitoring of wind turbine blades in order to estimate the level

of a critical damage as the initial stage before collapsing its. The wind turbine blade can split into three main regions.

Each region has a different multilayered structure, however, in each case the main defects are disbanded and delamination

and porosity [3&4].

The turbine blades consist of glass fiber reinforced plastics (GFRP) , gelcoat and sandwich areas containing

plastic foam covered with layer GRP[3].Nowadays it is common to use non-contact ultrasonic through transmission

techniques like air-coupled ultrasound to detect the inner damages [5].

The Reflection Coefficient

The reflection coefficient can be defined as a ratio between the magnitude of the incident wave propagation

towards the discontinuity and the reflected wave travelling in an opposite direction. Firstly, to obtain this we need to

calculate the acoustic impedance of material by law [2].

The characteristic impedance is defined as:

Z =ρ.c (1)

Where, Z is the characteristic acoustic impedance

ρ is the density of the medium

c is the compression\shear wave speed

Then is easy to calculate for every material by using this property and the reflection coefficient is evaluated in the

equation below between two different mediums.

Ґ= (Z2-Z1)/ (Z2+Z1) (2)



Damage Detection Using a Non-Destructive Technique for Inspection Wind Turbine Blades 31

From the equation above the magnitude of this reflected waves depends upon how different the values of Z2, Z1

are the more different they are higher the percentage of the wave’s energy is reflected back. The sample consists of two

layers; the top gel coat layer may vary in thickness between 1 mm and 3mm. The second layer is glass reinforced plastic

GRP and is 4 mm in thickness throughout.In NDT testing it is necessary to measure gelcaot slab, the far boundary will be between the gelcaot and the surrounding air.

Gelcoat: Compression velocity: 2608.00 m\s

Density: 1100.00 kg\m*3

Z shoe: Compression velocity: 2324.00 m\s

Density: 1077.00 kg\m*3

Now to calculate the characteristic acoustic impedance using:

Z =ρ.c

Zgel= 2608*1100 =2.868*10^6

zshoe=2324*1077 =2.502*10*6

Ґ= (2868800 - 2502948) / (2868800 + 2502948) = 0.146.

The magnitude of the reflection coefficient is 0.146 the reflected wave has a smaller amplitude. Moving to a wave

which is transmitted into the further medium at the boundary between GRP and the top gelcoat, some of the wave`s power

reflects back the way it came and some propagated through the boundary. It follows that mathematicians about the second

layer GRP could be situated adjacent to gelcoat layer. Again to find the reflection coefficient between them the velocity

and the density of the material GRP is shown below:

Compression Velocity: 2603.00 m\s

Density: 3500 kg\m*

Now to calculate the characteristic acoustic impedance using:

Z =ρ.c

ZGRP=26O3*3500= 9.110*10^6

Zgel = 2608*1100 = s 2.868*10^6

Ґ=( 9.110-2.868)\(9.110+2.868) = 0.52

The transmission will be:T=1- Ґ= 1-0.52=0.48

From this calculation we can notice that at this boundary between gelcaot and GRP 52% of the wave power is

reflected and 48% of the wave is transmitted.

The Propagation of Ultrasonic Waves

In solids, sound waves can propagate in four principle modes that are based on the way the particles oscillate.

Sound can propagate as longitudinal waves, shear waves, surface waves, and in thin materials as plate waves. However,

Longitudinal and shear waves are the two modes of propagation most widely used in ultrasonic testing. The particle

movement responsible for the propagation of longitudinal and shear wave is illustrated as shown in figure (2) below.




Figure 2: Graphical Representation of the Generation of Compression and Shear Waves

In Longitudinal or compression waves the movement of the particle is in the same direction as the wave

propagates this because as the particle a compress against its neighbor energy is transferred from particle to particle and

this continues along the wave.

In Shear or transverse waves the particle movement is perpendicular to the wave’sPropagation, this type of waves

is only present in acoustically solid material and does not propagate through liquid or gases. However during this project

all waves propagate into the sample at an angle of incidence 90 degrees. This means only one type of wave can generate by

the transducer at any one time. Also, after simulation the data need to be presented and interpreted to give information into

the nature of the sample. This is achieved by three ways, called A-scans, B-scan and C-scans. However, the particular

scanning technique is used in this project A &C Scans.

A- Scan

The A-scan plots received signal amplitude against time, with energy plotted on the Y-axis of the data produced

by single ultrasound measurement at a single point; the amplitude of a peak indicates the amount of energy received by the

boundary and the higher reflection coefficient the more energy that is received back by the transducer.The location of the

peak in the X-direction gives the user information into the depth of the defect; as the nearer peaks are closer the transducer.

The location of defect is using time of flight. Calculation can be made about layer thickness.

The voltage represented sound amplitude against time for all various echoes which have different propagation

paths through the test object depended on the reflection and transmission coefficient of each layer. Also, it is possible

defect defects by data analysis or comparison with ideal data. However, an A-scan gives information at one point only.

Defect Types

There are different types of defects can observe from ultrasound pulse – echo techniques when are sensitive to

them and see its location within the sample.

Disbond \ Delamination (Front\Back Face)

This is defined as an air gap between two materials which are caused during the production or applied stress

inducing a disbond. They occur in any material that has several layers and will have an effect on the integrity and strength

of the material.

Denominations are becoming more important to detect. This failure can be caused by a number of reasons;

material fatigue, impact, repeated temperature changes etc.

Front face refers to the air gap on the tester’s side of the boundary and back face refers to delamination on the

further side of the boundary [1, 2].




Porosity

This is a series of very small bubbles throughout the material which will cause energy to be lost in the propagation

ultrasonic wave. This may occur due to incorrect solidification of the adhesive. The energy loss is due to the sound wave

being scattered by the bubble[6].

Data Measurement and the Result

These tests have yielded important insights into technique for experimental damage detection on real structures

from using simulation package and material property. The method of interpretation set out in this section is very useful to

identify peaks from ultrasonic tests. In this paper analysis each defect type by calculating each individual path through the

sample. The basis of ultrasonic testing is to convert electrical energy into acoustic and vice versa by using a transducer

generally have a center frequency depend on the methods or structure being used; lower frequencies have longer time

duration pulses so are not suitable for testing thin layers.

Pulse-echo results from a transducer10 MHZ and shoe in contact with a sample, the main challenges when

interpretation their A-scan is matching the A-scan echoes with its corresponding propagation path through testing sample.

The diagram below shows 4 potential echo paths through a 2- layered for a sample section of gelcoat and GRP

(glass-reinforced plastic): the top gelcoat layer may vary in thickness between 1mm and 3mm. the second layer is

GRP and 4mm.

The first echo (1) is a pulse reflected from the buffer\shoe and the top of the gelcoat, the second echo is the pulse

between the gelcoat and the GRP. Echo (3) is the second reverberation in gelcoat. Echo (4) back face echo.By calculating

the time each echo takes to return to transducer, and the values for compression wave speed as below:

Gelcoat 2608

GRP 2603

A simple calculation can be performed to find thickness as below:

Thickness= 0.5(ΔT* Speed) (3)

The expected values are shown within the table below depended on the variable values of Δt gel &ΔtGRP to find

the thickness of gel coat and GRP.

Table 1: Thickness Gel Coat and GRB

Type Δt gel ThickerGel mm

Δt GRP ThicknessGRPmm

Type 1 0.00000122-0.00000046 1.004 4.20*10^-6-0.00000121 3.891 (4mm)

Type 2 0.00000123-0.00000046 1.017 1.99*10^-6-0.00000123 0.989 (1mm)

Type 3 1.99*10^-6-0.00000045 2.008 4.98*10^-6-1.98*10^-6 3.904 (4mm)

Type 4 1.22*10^-6-4.5*10^-7 1.004 3.49*10^-6-1.21*10^-6 2.967 (3mm)

Type 5 1.94*10^-6-0.00000045 1.916 -----------

Type 6 0.00000155-0.00000045 0.938 ------------

Type 7 0.00000118-0.00000046 1.434 -------------

By plotting the pulse-echo data which is supplied by the client, 36 A-scans but only seven different A-scan types

are explained.

The First Type: It includes six A-scans (A1, A5, B1, D1, D2 and E2) and each is plotted as in figure (3), the first

echo represents the top surface of the gel-cot and clearly has a low amplitude because of the similarities between the shoe




and the gel-coat (the both acoustic impedances are approximately similar as explained before, the waves transmit because

the reflection coefficient is( 0.146). The second echo, the back surface of the gel-coat and it has obviously a relatively high amplitude due to the fact

that the GRP existence in the second layer and the different properties between the gel-coat and the GRP, so the reflection

coefficient is (0.52).

Figure 3: The First Type Plot of A-Scan

From the material (transducer shoe, gel-coat and GRP) property data which is also provided by the client and is

plotted in figure (4), (5) and (6) respectively, the density and wave velocity in all three materials can be obtained with no

much change in the wave velocity corresponding to the frequency. Consequently, velocities of 2324 m/s, 2608m/s and

2603m/s in the shoe, gel-coat and GRP are chosen.

Figure 4: Wave Velocity in Transducer Shoe with Respect to Frequency

Figure 5: Wave Velocity in Gel-Coat with Respect to Frequency

Figure 6: Wave Velocity in GRP with Respect to Frequency

From the first and the second echoes, which come from the top and the back surface of the gel-coat, the thickness

of the gel-coat from the table above:Where, t1 is the time when the first echo arrive and t2 is the time when second echo

arrives,




Therefore, d1 ≈ 1 mm( thickness gel coat)

Echo (2), which is existed, as mentioned before, due to gel coat to the GRP, is not inverted, no disbond is

discovered. Moreover, the existence of a disbond makes the echo invert because the acoustic impedance of the air is much

lower than the gel-coat. To interpret the third and the forth echoes well, the time of the expected backface echo from the

GRP should be calculated and this can be done by using equation (3):

dgrp=0.5 (vgrp* tbf) (4)

Where, t bf is the time in which the wave travels through the GRP and comes back .

Therefore,

t bf = (2*dgrp)\ vgrp (5)

The time when the expected backface echo arrives = t2 + t bf (6)

This is the same time of the forth echo in the plot in figure (3) which means that this echo is the back face echo

from the GRP and the smaller shape is due to the attenuation in the GRP, as shown figure (7).

Echo(3), the second reverberation in gel coat. Furthermore, echo (4) occurs therefor energy goes into GRP.

Figure 7: Attenuation in GRP with Respect to Frequency

The Second Type: This type contains five identical A-scans (A2,A3,A4,B2 and B3), figure (8) shows the plot.

Figure 8: The Second Type Plot of A-Scan

The second type plot indicates that the first two echoes are almost the same as the previous type echoes.

Consequently, the gel-coat thickness is:

d2 = 2608x (12.3x10-7

-4.6x10-7

) / 2 =1.017 mm

The third echo has an amplitude compared to the one in the first type (which is taken as a reference as no

defect is existed), so it could not be a reverberation to the gel-coat. Furthermore, the fourth and fifth echoes do not appear

in the first type. As a result, the last two echoes have to come from the GRP layer and as they are equally spaced, this

shows that they might be multiple reverberations in the GRP.




In addition, the explanation for the large echo is due to the constructive interference in the GRP which means two

or more echoes arrive at the same time and the back face echo for the GRP could be one of them [2,7].

The Third Type: This type includes nine A-scans (A6, B5, 6, C4-6, D4, 5, and E3,5), each one is plotted as in

figure (9).

Figure 9: The Third Type Plot of A-ScanThe echoes in this type are similar to the ones in the first type except the fourth echoing, the thickness of the

gel-coat (d3) =2mm and the time of the expected backface echo from the GRP (t bf ) = 49.8x10-7

s.

If blackface is much lower amplitude by comparing with a-perfect sample it is easily noticeable that a small peak

is visible because of the nature of porosity that is defined as a series of very small air bubbles throughout the material

which will cause energy to be lost by the propagation ultrasonic wave. The scattered waves appear as random noise

imposed on the front and back so that the amount of wave energy that is scattered depends on the size of the scattering

particles compared to the wavelength of the ultrasonic wave. However, porosity is not determined during this inspection.

The Fourth Type: This type includes five A-scans (B4,C1-3,D3) each one is plotted as in figure (10).

Figure 10: The Fourth Type Plot of A-ScanThe echoes in this type are similar to the first type except the fourth echo.

As the thickness of the gel-coat (d4) =1mm and the time of the expected backface echo from the

GRP (t bf ) = 34.9x10-7

s.However, there is no echo occurring at this time while the fourth echo arrives before this time and

exactly in the second of 12.1x10-7 which indicates that a defect (delamination) is discovered.The depth of the delamination

is determined by applying equation (3).

And from the table of thickness above:Therefore, the depth of delamination =2603x22.8x10-7 / 2 ≈ 3

This is mean that the delamination is at 3 mm from the front surface of the GRP and at 4 mm (1mm + 3mm) from

the surface of the gelcoat.

The Fifth Type: In this type five A-scans (D6.E6,F4-6),each is plotted as shown in figure (11).

Thickness of the gel-coat is also obtained from the table above:




d5= 2608x (12.3x10-7

- 4.5x10-7

) / 2 = 1.916 mm.However, the inversion of the second echo besides the high

amplitude which all indicate that a disbond is existed between the gel-coat and the GRP.

Figure 11: The Fifth Type Plot of A-Scan C-Scan

C-scan is just the product of several A-scan over a large area whereas, A- scan only describe the sample at one particular point. C-scans give a very nice a comprehensive three dimensional picture of the entire sampling, easy to

analysis the shape of the defect to the operator by color map of part of a structure.

A-scan e.g. arrival time of the particular peak which would give layer thickness at the point or maybe the

amplitude of a particular peak, which might give an indication of attenuation to form a C-scan a series of A scan

measurements for example using the pulse echo technique across the surface area of the test object. This is done in a

logical grid formation .So for each measurement position there will be an A- scan.

As shown in figure 12, the layer thickness can put into grid representing the test object. It is normal then to color

code this information in the form of color contour map. Here we can see the thickness of a delicate layer change from thinin lower left hand corner test object to thicker in the right hand corner [7].

Figure 12: C-Scan Format

Another type of C-scan as is shown in figure (13) to detect disbonds and delaminations. It is the amplitude data

will be most useful generally, the pulse will be transmitted into the top substrate layer and some energy will then be

transmitted into the second layer. However, a disbond is where a gap between them this is usually a very thin air gap no

energy into GRP. Red disdond, green OK and thickness of grp is less 4mm equal delamination (yellow) [7].

A B C D E F

1 3mm

2 1mm 1mm 3mm

3 1mm 1mm 3mm 3mm

4 1mm 3mm

5

6

Figure 13: C-Scan Disbond(Red) & Delamination(Yellow)




CONCLUSIONS Pulse – echo measurement technique of measurements the ultrasonic waves by using the PROMAT and the

characteristic of these graphs to identify defects in sample of wind turbine blade structures. It was possible to investigate

the propagation characteristic of all the different defects set out in theory and the method of interpretation which is useful

to identify the defects including disbonds and delaminations which were presented and indicated. IN the future, it would

possible to generate this technique allowing quicker analysis of each A-scan. That any person wishing to understand

industrial ultrasonic waves. This technique for the inspection of wind turbine blade has the advantages to work during the

tests. However, the influence of overlapped reflections, scattering and attenuation of the reflected ultrasonic waves from

multi-layered structure takes place. This effect has a negative impact on the propagation of ultrasonic waves.

REFERENCES

1. Shull, Peter, J.,ed. Nondestructive Evaluation: theory, techniques, and applications. New York: Marcel

Dekker, 2002.

2. Non- destructive Testing Encyclopedia.http://WWW.ndt,net/article/az/ut/apped a/append a, htm

3. Jasiūnienė, E., et al.Ultrasonic NDT of wind turbine blades using contact pulse -echo immersion testing with

moving water container [online]. Kaunas: ULTRAGARSAS, 2008.

4. Yolken, H., Thomas , Matzkanin, George, A., and Bartel Jill E. Nondestructive Evaluation (NDE) of Advanced

Fiber Reinforced Polymer Composites [online]. Texas: Nondestructive Testing Information Analysis Center

NTIAC, 2001.

5. Jasiūnienė, E., et al.Ultrasonic NDT of wind turbine blades using contact pulse-echo immersion testing with

moving water container [online]. Kaunas: ULTRAGARSAS, 2008.

6. Torres-Sanchez, C., Corney, J. R. Porosity tailoring mechanisms in sonicated polymeric foams [online]. Glasgow:

IOP Publishing Ltd, 2009.

7. Manwell, J., Roger, A.,Wind energy explained theory and application.2nd

ed.., Chichester: Wiley, 2002.

4. comp sci - ijcse - damage detection using a non-destructive maysoon h. al-moswi iraq

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